Weird Error Upgrading ASP.NET MVC from 4 to 5

I'm converting my project from MVC 4 to MVC 5 (and .Net 4 to .Net 4.5.2, which is the real driver of the changes.)

When I run one of my pages I get this error (blank space added by me for easier reading)

[A]System.Web.WebPages.Razor.Configuration.HostSection cannot be cast to [B]System.Web.WebPages.Razor.Configuration.HostSection.

Type A originates from 'System.Web.WebPages.Razor, Version=1.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' in the context 'Default' at location 'C:\windows\Microsoft.Net\assembly\GAC_MSIL\System.Web.WebPages.Razor\v4.0_1.0.0.0__31bf3856ad364e35\System.Web.WebPages.Razor.dll'.

Type B originates from 'System.Web.WebPages.Razor, Version=3.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' in the context 'Default' at location 'C:\Windows\Microsoft.NET\Framework\v4.0.30319\Temporary ASP.NET Files\studentportal3g\2204bad2\aece9b3b\assembly\dl3\ad80387c\91adbf51_fc73d101\System.Web.WebPages.Razor.dll'.

When I first saw this is though, Ah easy! Not so much :)

I've gone over every project and made sure it's version is upgraded to MVC 5 which has the 3.0.0.0 version of System.Web.WebPages.Razor.dll.

Clean rebuild, still get the error. No problem , I'll delete the cached temp files.

Clean rebuild, still get the problem. I go back, manually check each version of System.Web.WebPages.Razor.dll, in the references of each project that has a reference to it. I check my folder where I copy dlls to make references to them manually, it's not there.

If my solution doesn't' contain a copy of the DLL or a reference to the DLL, and I've manually deleted the cache folders in 'C:\Windows\Microsoft.NET\Framework\v4.0.30319\Temporary ASP.NET

Files\studentportal3g...

Where is the old bad dll coming from? How do I fix this error? How do I prevent it happening again?

Thanks,

Eric-

Visual Studio is a great tool, but it doesn't always make the right choices when it comes to upgrading dependencies, nor does it support every possible option available in MSBuild. Whenever you find yourself in a bind such as this you should manually review and (if necessary) edit your .csproj file in order to resolve it.

The problem isn't that your file exists in the GAC or that it has not been installed by NuGet, the issue is most likely that one of your project files still has a reference to the old version of System.Web.WebPages.Razor version 1.0.0.0, and you need to find all references to it and change them to 3.0.0.0 accordingly.

1. Right-click on your project node in Solution Explorer and click Unload Project.
2. Right-click the project node again and click Edit <projectName>.csproj.
3. Search the file for references to System.Web.WebPages.Razor and update the version and the HintPath accordingly (as shown below). Make sure the HintPath you use actually points to an existing file.
4. Repeat these steps for all dependent projects in the solution (and any that are in DLLs that are not part of the solution).

Old Reference

<Reference Include="System.Web.WebPages.Razor, Version=1.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35, processorArchitecture=MSIL">
    <Private>True</Private>        
    <HintPath>..\packages\Microsoft.AspNet.WebPages.1.0.20105.408\lib\net40\System.Web.WebPages.Razor.dll</HintPath>
</Reference>

Updated Reference

<Reference Include="System.Web.WebPages.Razor, Version=3.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35, processorArchitecture=MSIL">
  <Private>True</Private>
  <HintPath>..\packages\Microsoft.AspNet.WebPages.3.0.0\lib\net45\System.Web.WebPages.Razor.dll</HintPath>
</Reference>

You should also go through the web.config and /Views/web.config files to ensure that they are not referencing any old versions of this assembly.

NOTE: If the above instructions don't solve your issue, the issue likely is outside of your solution. Most likely there is a 3rd party library that is referencing the old version of the file somewhere. If so, you could attempt to get an updated version of the DLL.

You may also want to check out this question.

It looks like the old DLL is in the Global Assembly Cache (GAC). The GAC is a place where you can store assemblies that can be referenced from several applications on the machine. Click here to read more about GAC.

Use the tool gacutil to update the assembly in the GAC.

The tool is located somewhere under "Microsoft SDKs" folder in Program Files. For me, it was located in

C:\Program Files (x86)\Microsoft SDKs\Windows\v10.0A\bin\NETFX 4.6.1 Tools\

Open cmd and navigate to the place where gacutil i stored. Then use it like this:

gacutil.exe -i [path to your assebly] -f.

The -i parameter is for indicating where your assembly is located. The -f parameter is used to force an update of the assembly, if it was already there.

Example

Say your DLL is located in

C:\temp\System.Web.WebPages.Razor.dll

Then you would run gacutil.exe -i "C:\temp\System.Web.WebPages.Razor.dll" -f